Lake Wohlford Dam stable … unless big earthquake hits

The then-76-foot-high dam was formed by granite blasted from the
hillsides during the construction of the flume, which carried water
from the San Luis Rey River 14 miles away. One of the roughly 75
men who worked to build the dam died in a premature explosives
discharge, according to Escondido historian Frances Beven Ryan.

The total cost at the time was about $450,000, including the
construction of the flume.

Redwood planks were used to line the lake-facing side of the
dam, with tar sealing the wood like on a ship's hull. The dam was
strong enough to survive torrential rains in January 1916, which
sent water from Bear Valley Lake flowing over the dam.

In 1922, the Escondido Mutual Water Co. decided to raise the dam
to hold additional water from recently created Lake Henshaw,
doubling the lake's capacity. Engineer Harry Hawgood recommended
the then-common technique called "hydraulic fill": essentially,
packing mud behind the original dam.

Photographs of the construction from the Escondido Historical
Society show a pipe conveying a slurry from the lake bed into a
pond between the original dam and a mound behind it. As the solids
settled, water was pumped out.

The now-100-foot-high, 420-foot-wide dam, renamed after
long-serving water company board member Alvin Wohlford, was
completed in 1924 with a maintenance road across the top. The dam
has a volume about two-thirds of the Transamerica building in San
Francisco.

What to expect from Elsinore

Engineers don't construct hydraulic fill dams anymore because
they're not as strong as such modern materials as rolled
concrete.

Still, city plans say the Lake Wohlford Dam is strong enough to
withstand "peak ground acceleration" of up to 0.27 g, or just over
a quarter the strength of gravity.

That's equivalent to shaking from a magnitude-7.3 quake from the
Elsinore fault. In recorded history, the Elsinore fault, which runs
through Julian and close to Lake Henshaw, has never produced a
quake of that magnitude, according to the Southern California
Earthquake Center.

The last major rupture was in 1910, generating a magnitude-6
quake. An extension of the fault in Mexico did produce a
magnitude-7 quake in 1892.

"The Elsinore fault is not inactive," said Tom Rockwell, a
geology professor at San Diego State.

However, a large earthquake —— magnitude 7.5, for example —— is
probable roughly every 2,000 years on the Elsinore fault, he
said.

That compares to every 100 to 200 years on the more distant, but
more active San Andreas or San Jacinto faults, Rockwell said.

Escondido officials would have had to replace or reinforce the
Lake Wohlford Dam more than 20 years ago if the Elsinore fault were
more seismically active.

In 1982, the state Division of Safety of Dams told the city of
Escondido that it had to prepare for a magnitude-7.5 quake. That
would have required the city to spend millions to strengthen the
dam or keep the reservoir level lower.

State officials backed off after Nevada earthquake expert D.B.
Slemmons advised them that a 7.5-sized quake from the Elsinore
fault was only possible if most of the fault ruptured at the same
time. This was considered unlikely because of the fault's twists
and turns.